Morris—The Preparation of Selenic Acid 
373 
The concentrated acid is drawn from the retort into a large 
wash bottle, care being taken to avoid drawing air through 
the solution. Well .washed hydrogen sulfide is next passed 
through the acid until precipitation is complete, and the pre¬ 
cipitate has become floeculent. The excess of hydrogen sulfide 
is then blown out by a stream of air from which all dust has 
been removed by bubbling through concentrated sulfuric acid. 
The hydrogen sulfide serves to remove any selenious acid 
which may be present, decomposing it with the precipitation of 
sulphur and selenium. This precipitate is removed by filtering 
through an asbestos felt. 
It is frequently stated that selenic acid is not acted upon by 
hydrogen sulfide, but after each concentration of the acid, 
hydrogen sulfide produces a light precipitate. That this is 
not due to the decomposition of selenic acid by heat is shown 
in the following manner: Selenic acid free from selenious acid 
and giving no precipitate with hydrogen sulfide is concentrated 
at room temperature in a vacuum desiccator over sulfuric acid 
or sodium hydroxide. This acid as it becomes more concen¬ 
trated will give a precipitate with hydrogen sulfide. 
By these experiments it is shown that the method of J. Thom¬ 
sen will give excellent selenic acid if particular attention is 
given to certain details of the process. 
